As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has been also sharply increased as a power source for the mobile devices. One of the secondary batteries is a lithium secondary battery having high energy density and discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Based on its external shape, the lithium secondary battery is classified as a cylindrical battery, a rectangular battery, or a pouch-shaped battery. Based on its electrolyte, the lithium secondary battery is classified as a lithium-ion battery or a lithium-ion polymer battery.
As the mobile devices have been miniaturized, the demand of the rectangular battery and the pouch-shaped battery, which have a relatively small thickness, has increased. The typical shape of a battery pack, which is one example of such a small-thickness secondary battery, is shown in FIG. 1. FIG. 2 is an exploded perspective view illustrating the battery pack before assembly.
Referring to FIGS. 1 and 2, the battery pack 100 comprises: a rectangular core pack 200 having an electrode group, which includes a cathode, an anode, and a separating film, and an electrolyte mounted therein in a sealed state; a case body 300 having an inner space for receiving the core pack 200; and an upper cover 400 mounted on the case body 300 having the core pack 200 received therein for sealing the core pack 200.
Generally, the assembly of the battery pack 100 having the above-described structure is accomplished by coupling the upper cover 400 and the case body 300, which are made of a plastic material, such as polycarbonate (PC) or polyacrylonitrile-butadiene-styrene (ABS), to each other by an ultrasonic welding method. The ultrasonic welding method is a method of welding two surfaces to be attached using frictional heat generated by high-frequency vibrations, for example, 20,000 Hz.
Coupling the case body 300 and the upper cover 400 by the ultrasonic welding method will be described in more detail with reference to FIGS. 3 to 6. FIG. 3 is a plan view illustrating the upper cover 400 mounted on the case body 300, and FIG. 4 is a vertical sectional view taken along line A-A of FIG. 3. While the upper cover 400 is mounted on the case body 300, opposite ends of the upper cover 400 are in contact with opposite ends of the case body 300, respectively. FIG. 5 is an enlarged view illustrating the contact area B between the upper cover and the case body, and FIG. 6 is a view illustrating coupling the upper cover and the case body by the ultrasonic welding method. As shown in FIG. 5, a wedge-shaped weld protrusion 400a is formed at the lower surface of each end of the upper cover 400, and a weld surface 300a is formed at the case body 300 such that the weld protrusion 400a is brought into contact with the weld surface 300a. When high-frequency vibrations are applied to accomplish the ultrasonic welding between the upper cover and the case body, the weld protrusion 400a and the weld surfaces 300a are welded, and therefore, the upper cover is securely coupled to the case body.
As the demand of smaller-thickness battery packs has increased, the thickness of the case body 300 and the upper cover 400 has been recently decreased to 0.3 to 0.35 mm. As a result, it is difficult to form the case body and the upper cover by die casting and injection molding. Furthermore, the sizes of the weld protrusion 400a and the weld surface 300a are also decreased, and therefore, the welding strength between the case body and the upper cover is reduced, thereby increasing a welding failure rate.
In the case of a battery using a cylindrical metal case, the case provides an appropriate strength against external impacts due to the structural characteristics of the metal case even when the thickness of the case is small. However, a lithium-ion polymer battery having the structure as shown in FIG. 1 does not have an appropriate structural strength against external impacts, and therefore, use of a small-thickness case in the lithium-ion polymer battery is restricted.
Consequently, it is necessary to provide a battery pack which has an appropriate strength against external impact even when a small-thickness case is used and which has improved coupling strength between the upper cover and the case body.